The invention relates to a method for identifying gaseous compounds and determining their concentration in a gas mixture by analysis of an emission spectrum generated by rotational transitions of molecules in gaseous substances exposed to microwave irradiation and to an apparatus for performing the method.
It is known that the composition of gas mixtures, but also of liquids and solids which can be converted to gaseous states in a suitable manner, can be quantitatively determined by microwave irradiation. Irradiation of gases by microwaves of a suitably selected frequency stimulates the molecules of a respective one of the gases in the gas mixture to be analyzed to undergo rotational transitions which are typical for the molecule such that knowledge of the rotation frequencies of various gases permits their identification in the gas mixture.
In a method and apparatus disclosed in DE-OS 3622957, the absorption of frequencies from the incident microwave radiation is utilized for analysis purposes. Herein the absorption lines are split by application of an electric field resulting in the "Stark" effect which provides for a relatively good analysis sensitivity because of the Stark effect modulation. It is also known that the width of the absorption lines can be reduced and consequently the resolution of the measuring method can be improved if the analysis is performed in a space maintained at reduced pressure.
In this method it is considered to be disadvantageous that, for performing the analysis, which is also called "Stark-spectroscopy", it is generally necessary that the molecules of the gas to be analyzed have electric dipole moments of a strength of at least 0.2 Debye. Also the quantitative determinations depend greatly on the pressure and temperature in the measuring space which therefore need to be exactly determined and controlled but still they may be sources for incorrect measuring results.
The magazine "Review of Scientific Instruments", Vol. 52, No. 1, 1981, pages 33-45 discloses a measuring method, similar to the one with which the present invention is concerned, for investigating the resonance transitions of short-lived molecule structures. The apparatus utilized in this method is a microwave spectrometer in which the particles of a gas added to an inert carrier gas under approximately atmospheric pressure are admitted to an evacuated receiver by way of a preferably pulsed admission valve and are conducted through a microwave field which is generated in a Fabry-Perot resonator. Since the expansion of the gases in the receiver is an adiabatic process, the gas temperature is lowered which facilitates the investigation, by means of microwave irradiation, of molecules which are added to a gas used as a carrier gas. The rotation lines of low rotational quantum numbers to be observed are intensified and complicated rotation spectra with a large number of lines are substantially easier to analyze by elimination of lines with high rotational quantum numbers.
It is the principal object of the present invention to provide a method and apparatus adapted to permit accurate quantitative determination of the concentration of a gas in a gas mixture. The method and apparatus should provide for high detection efficiency with good definition and, as a result, little interference. The method and apparatus should be usable for the analysis of a large number of gas mixtures particularly in the control of environmentally noxious compounds.